1. Field of the Invention
The invention relates to a circulating fluidized bed boiler for combusting wastes or solid fuels which contain corrosive components such as chlorine, by feeding the wastes or the solid fuels together into circulating fluidized bed in a furnace.
2. Background Art
FIG. 5 shows a construction of a conventional circulating fluidized bed boiler. Generally, the circulating fluidized bed boiler comprises a furnace 2, a cyclone dust collector 3 into which flue gas which is generated by the combustion in the furnace 2 flows and which catches particles which are contained in the flue gas, a seal box 4 into which the particles which are caught by the cyclone dust collector 3 flow and external heat exchanger 6 which performs heat exchange between the circulating particles and in-bed tubes in the heat exchanger 6.
The furnace 2 consists of a water cooled furnace wall 2a and an air distribution nozzle 7 which introduces fluidizing air A to the furnace 2 so as to create a fluidizing condition in the furnace 2 is arranged in a bottom part of the furnace 2. The cyclone dust collector 3 is connected with an upper part of the furnace 2. An upper part of the cyclone dust collector 3 is connected with the heat recovery area 8 into which flue gas which is generated by the combustion in the furnace 2 flows, and a bottom part of the cyclone dust collector 3 is connected with the seal box 4 into which the caught particles flows.
A super heater and economizer etc. contain in the heat recovery area 8.
An air box 10 is arranged in a bottom of the seal box 4 so as to intake upward fluidizing air B through an air distribution plate 9. The particles in the seal box 4 are introduced to the external heat exchanger 6 and are in-bed tube 5 under fluidizing condition.
In the furnace of the above explained circulating fluidized bed boiler, bed materials 11 which comprise ash, sand and limestone etc. are under suspension by the fluidizing condition.
Most of the particles entrained with flue gas escape the furnace 2 and are caught by the cyclone dust collector 3 and are introduced to the seal box 4. The particles thus introduced to the seal box 4 are aerated by the fluidizing air B and are heat exchanged with the in-bed tubes 5 of the external heat exchanger 6 so as to be cooled. The particles are returned to the bottom of the furnace 2 through a duct 12 so as to circulate through the furnace 2.
In the above conventional fluidized bed boiler, corrosion on the high-temperature area of the in-bed tubes 5 tends to occur due to chlorine which is contained in the particles.
This is because the circulating particles contain unburned fuel which contains a chlorine and combusts in the seal box 4 together with the fluidizing air B. The unburned fuel thus combusted in the seal box 4 generates melted salts which contain sulfate and condense so as to adhere to a high temperature area in the heat exchanger 6. Further, a high temperature corrosion by corrosive halogen gas, e.g., chlorine gas, which is generated during the above combustion occurs in the heat exchanger 6.